Christopher Wren's St Paul's Cathedral is a masterful piece of engineering that solves the basic contradiction of dome architecture — a dome that looks well-proportioned inside is usually too low outside, while a dome that is magnificent outside is too tall and dark inside — with three separate shells.

Instead of a single dome Wren designs three nested layers. Innermost is a low brick inner dome facing the interior, its proportion preserved, with an opening (oculus) at its crown. Outermost is the much taller, bulbous timber-framed, lead-clad outer dome that dominates the London skyline. But the real genius lies between them: an invisible, self-concealing brick cone. This cone is the true structure carrying the heavy stone lantern at the top; neither the thin inner dome nor the light outer timber shell could bear that load. The two visible domes are aesthetic, the unseen cone structural — form and structure are deliberately separated.

Wren had trained as a mathematician and astronomer; he solved the dome as a problem of geometry and statics. This three-shell system refines the hidden logic of St Peter's in Rome and many later great domes, and defines the measured, engineering-minded character of the English Baroque, distinct from the exuberant curves of the Continent. St Paul's is the canonical solution to reconciling the demands of what a dome 'must look like' and what it 'must do to stand', by dividing them into separate layers.